Hello- I have a 1 year old son. I would love to have a daughter next. Is there anything that you know of that I can do to push the odds of having a girl???

-A curious adult from New York

People have probably been trying to pick the sex of their kids since time began. For example, in the Middle Ages, men who wanted a boy sometimes had their left testicle removed. This was because people believed that the right testicle made "boy" sperm and the left made "girl" sperm.

None of the various high and low-tech methods of today that claim to be able to influence whether you'll have a boy or a girl are quite so drastic. Since you want a girl, a method called MicroSort is available that can increase your chances of having a girl to around 90%. Of course, it isn't free.

Many methods (including MicroSort) take advantage of the fact that "boy" sperm has less DNA than "girl" sperm. Remember, the gender of your baby is determined by whether the fertilizing sperm has an X or a Y chromosome. An X chromosome will lead to a girl, a Y to a boy.

Why do "boy" sperm have less DNA? Because the Y chromosome is so much smaller than the X chromosome. The only real function of the Y chromosome is to eventually turn the fetus into a fertile male. The X on the other hand, has 1000's of genes involved in all sorts of things.

The most popular low-tech method that tries to take advantage of the lower amount of DNA in Y sperm is called the Shettles approach. The idea behind this method is that Y sperm are faster and more fragile than X sperm.

The Shettles method recommends that if you want a girl, plan to make a baby right around the time of ovulation. At this time, the egg is as far away as possible from the incoming sperm so the distance runners of the sperm world, the X sperm, have a better chance of making it to the egg.

For a boy, plan to make a baby about 2-4 days after ovulation. That way the sprinter Y sperm can make it to the egg first. Sounds interesting and seems to be based on reasonable science, but as I said, I haven't seen any reliable evidence vouching for this method.

MicroSort also uses the fact that Y sperm have less DNA. MicroSort separates the X and the Y sperm and only uses one kind to fertilize the egg(s). The fertilization can take place either in a dish or, more "naturally," by putting the sperm directly into the mother-to-be. (As you'll see below, the dish way is much more invasive and expensive.)

MicroSort cannot separate X and Y sperm with 100% accuracy. This technique has much better odds for getting a girl than a boy. The claims are 91% chance for a girl and around 76% for a boy. Looks like a good method for getting a girl if you want to cough up at least 3 or 4,000 dollars.

If you really want to break the bank, you can use a newer technique called preimplantation genetic diagnosis, or PGD. In PGD, eggs are fertilized outside of the womb and allowed to grow in a dish. Once they reach a certain size, a cell is taken and the scientist can look at its DNA and figure out if it will grow into a boy or a girl.

If you want a boy, you put back embryos with an X and a Y chromosome. If you want a girl, you put back embryos with two X chromosomes. Easy, huh?

Well, actually not. The method is incredibly invasive, expensive, and you have to be willing to discard fetuses of the wrong sex. In this procedure, the woman needs hormone treatments to stimulate making more eggs. The eggs then need to be harvested from the ovaries of the mother-to-be (much more difficult than harvesting sperm!).

Lots of embryos are created and only a few of the right sex are kept. The rest are frozen away and probably discarded eventually. The chances of putting either a boy or a girl fetus into the womb are pretty close to 100% with this method.

Of course, at 8-16,000 dollars, it had better be! Because it is so expensive, most people use this to screen for genetic or sex-specific diseases rather than just to pick the sex of their baby (although it is available for family balancing right now). In the future, if the price comes down, maybe more people will use it.

As you can see, a number of scientific methods are available. Of course, there are also the old wives' tales about the phase of the moon or the mother or father's diet (chocolate for girls, red meat for boys) or lots of others. While they may be fun to try, don't count on them working!

This project was supported by the Department of Genetics, Stanford School of Medicine. Its content is solely the responsibility of the authors and does not necessarily represent the official views of Stanford University or the Department of Genetics.